Photolithographic systems and methods for producing sub-diffraction-limited features

ABSTRACT

Systems and methods for near-field photolithography utilize surface plasmon resonances to enable imaging of pattern features that exceed the diffraction limit. An example near-field photolithography system includes a plasmon superlens template including a plurality of opaque features to be imaged onto photosensitive material and a metal plasmon superlens. The opaque features and the metal superlens are separated by a polymer spacer layer. Light propagates through the superlens template to form an image of the opaque features on the other side of the superlens. An intermediary layer including solid or liquid material is interposed between the superlens and a photoresist-coated semiconductor wafer to reduce damage resulting from contact between the superlens template and the photoresist-coated semiconductor wafer.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. applicationSer. No. 11/329,755, filed Jan. 11, 2006, entitled “PHOTOLITHOGRAPHICSYSTEMS AND METHODS FOR PRODUCING SUB-DIFFRACTION-LIMITED FEATURES,”which is hereby incorporated by reference herein in its entirety.

BACKGROUND

1. Field

The present teachings relate to photolithography systems and methodssuch as for use in fabricating semiconductor devices.

2. Description of the Related Art

Conventional “far-field” photolithography systems use light and a lenssystem to image a reticle having a pattern thereon onto a layer ofphotosensitive material deposited on a semiconductor wafer. Suchconventional photolithography systems are referred to as “far-field”systems because the image produced is in the far field. Accordingly, thesize of features in the reticle pattern that may be reproduced on thelayer of photosensitive material is limited by far-field diffraction.The minimum feature size W that may be produced using a conventionalfar-field photolithographic system is

$\begin{matrix}{W = {k_{1}\frac{\lambda}{NA}}} & (1)\end{matrix}$

where NA is the numerical aperture of the system, λ is the wavelength oflight used by the photolithography system, and k₁ is the resolutionfactor, which depends on other aspects of the system including, forexample, the aberrations introduced by the specific photolithographysystem and the properties of the photosensitive material. According tothis equation, to produce smaller features, the photolithographic systemmust utilize smaller resolution factor, a larger numerical aperture, asmaller operating wavelength, or a combination thereof.

Current far-field lithographic systems include complex lens that arewell-corrected for aberrations. Accordingly, current far-fieldlithographic techniques have decreased the resolution factor to k₁≈0.3,which is slightly greater than the theoretical lower limit of 0.25 forhalf-pitch imaging. These complex lens, however, may include manyoptical elements and are expensive.

The numerical aperture has also been increased. However, in systemswhere light propagates through air from the lens system to thesemiconductor wafer having photosensitive material thereon, thenumerical aperture is limited to one. Immersion lithography wherein thelight propagates through a medium having an index of refraction greaterthan one has lead to increases in NA. Immersion techniques, however,suffer from problems such as incompatibility of the fluid and the wafer,bubble formation, and polarization effects. Further increases in NA arelimited, however, because of the limited range of compatible immersionfluids having refractive indexes above one.

Smaller features may also be produced by using light sources havingshorter operating wavelengths. Commercial photolithography systems mayuse visible light having wavelengths in the range of 350 nm-800 nm.Ultraviolet photolithographic systems operating with wavelengths in therange of 100 nm-350 nm may be used to print smaller features.Ultraviolet systems, however, also suffer from drawbacks such asincreased cost, shorter lamp lifetimes, and lower efficiency.Furthermore, photoresist that is sensitive to visible light is cheaperand more robust with respect to airborne contaminants thanultraviolet-sensitive photoresist.

Thus, what is needed are photolithography systems that are notrestricted to the diffraction limit of far-field optical systems andneed not rely on use of complex and expensive lenses, immersiontechniques, or the use of ultraviolet wavelengths.

SUMMARY

A variety of different embodiments of the invention are disclosedherein. Some of these embodiments comprise photolithographic systemscomprising near-field optical systems.

One embodiment, for example, comprises a photolithographic system forexposing a photosensitive material having a first index of refractionand responsive to light having a wavelength, λ. The system comprises aplurality of features that are opaque to the light, a dielectricmaterial disposed forward of the plurality of opaque features, asuperlens disposed forward of the dielectric material and rearward ofthe photosensitive material, and an intermediary layer between thesuperlens and the photosensitive material. The dielectric material issubstantially transmissive to the light. The intermediary layercomprises material substantially transmissive to the light with thematerial being different than the photosensitive material.

Another embodiment comprises a photolithographic system for producing anear-field image. The system comprises a substrate that is substantiallytransmissive to light, the light having a wavelength λ. The systemfurther comprises one or more features disposed on the substrate, thefeatures being substantially opaque to the light, a spacer materialdisposed on the substrate, the spacer material being substantiallytransmissive to the light, and a superlens formed on the spacermaterial. The spacer material has a first complex permittivity with areal part that is positive, and the superlens comprises material havinga second permittivity with a real part that is negative. The systemfurther comprises a layer of photosensitive material that is inproximity to the superlens and an intermediary layer that is interposedbetween the superlens and the layer of photosensitive material such thatthe superlens is separated from the photosensitive material. The layerof photosensitive material has a first index of refraction, and theintermediary layer has a second index of refraction substantially equalto the first index of refraction. The system produces a near-field imageof the features in the layer of photosensitive material.

Another embodiment is a method of fabricating an integrated circuitdevice on a semiconductor wafer. The method comprises depositing amaterial to be patterned over the semiconductor wafer. A photosensitivelayer is deposited on the material to be patterned. A superlens templateis disposed in an optical path between a light source and thephotosensitive layer. The photosensitive layer has a first index ofrefraction and is responsive to light having a wavelength, λ. Thesuperlens template comprises a plurality of features substantiallyopaque to the light, a dielectric material disposed forward of theplurality of opaque features, and a superlens disposed forward of thedielectric material and rearward of the photosensitive layer. Thedielectric material is substantially transmissive to the light. Thephotosensitive layer has a first index of refraction and is responsiveto light having a wavelength, λ. The method further comprisesinterposing an intermediary layer between the superlens and thephotosensitive layer to reduce contact of the superlens with thephotosensitive layer. The intermediary layer comprises materialsubstantially transmissive to the light. The light is directed into thesuperlens template thereby exposing portions of the photosensitive layerto the light.

Another embodiment comprises a photolithographic system for exposing aphotosensitive material to a light beam. The system comprises means forblocking portions of the light beam to form a pattern of light and meansfor generating plasmons with the pattern of light. The system furthercomprises means for coupling the pattern of light into the plasmongenerating means and means for protecting the photosensitive materialfrom contact with the plasmon generating means. The coupling means aresubstantially transmissive to the light beam, and the protecting meansare disposed such that light couples out of the plasmons into theprotecting means.

An additional embodiment comprises a method of exposing a photosensitivematerial to a light beam. The method comprises blocking portions of thelight beam to form a pattern of light, propagating the pattern of light,and generating plasmons with the pattern of light. The method furthercomprises coupling light out of the plasmons into a medium substantiallyoptically transmissive to the light coupled out of the plasmons andprotecting the photosensitive material from contact using the medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a superlens template for an embodiment of anear-field photolithography system wherein the superlens templatecomprises opaque features (shown) and a metal superlens layer (notshown).

FIG. 2 is a cross-sectional view of an embodiment of the superlenstemplate taken through the line 2-2 in FIG. 1 that shows both opaquefeatures and the metal superlens layer.

FIG. 3 is a cross-sectional view of an embodiment of a near-fieldphotolithography system showing the superlens template disposed over aphotosensitive material and having an intermediary layer therebetween.

FIG. 4A is a schematic diagram of an embodiment of a near-fieldphotolithography system wherein the intermediary layer comprises acoating on the photosensitive material.

FIG. 4B is a schematic diagram of an embodiment of a near-fieldphotolithography system using a flowing fluid as the intermediary layer.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

Various embodiments of the invention comprise photolithography systemsthat use a superlens template 14, a top view of which is shownschematically in FIG. 1. This superlens template 14 comprises asubstrate 18 and a pattern 23 comprising one or more features 22 to beimaged by the photolithography system.

Images of the pattern 23 may be produced by propagating light throughthe superlens template 14. In certain embodiments, for example, thelight may be incident on an upper surface of the template 14. The lightmay have an operating wavelength λ. This wavelength may correspond to acentral wavelength of a wavelength band. This wavelength band may be anarrow band having a bandwidth (FWHM) of one nanometer or less or mayinclude one or more nanometers. Suitable operating wavelengths may be inthe ultraviolet and visible range of the electromagnetic spectrum fromabout 100 nm-800 nm, and may include, for example, the I-line of mercurylamps (λ=365 nm), krypton-fluoride excimer lasers (λ=248 nm), orargon-fluoride excimer lasers (λ=193 nm). Use of visible wavelengthoffers several advantages such as described above. However, otheroperating wavelengths of light may be used, including extremeultraviolet and infrared wavelengths.

Accordingly, in various embodiments, the substrate 18 is substantiallytransmissive to light of the operating wavelength and may comprisematerials such as glass or quartz. Other materials that aresubstantially transmissive to light of the operating wavelength may beused. The material comprising the substrate 18 may have a substantiallyflat surface produced by, for example, polishing or planarizationtechniques, although the surface may be other than flat.

Conversely, the features 22 may comprise material that is substantiallyopaque to the operating wavelength. Other characteristics of thematerial comprising the features 22 may include a low skin depth suchthat light is quickly absorbed into the material. The features 22 neednot be extremely thick to attenuate the light transmitted through thesubstrate 23. Additionally, the material may resonate at frequencies farfrom the plasmonic resonances of metals in the system. The materialcomprising the features 22 may also be able to wet and adhere to thesubstrate 18. Suitable materials include, for example, chromium,tungsten, titanium, titanium silicide, titanium nitride, siliconnitride, or their alloys or composites. In certain preferredembodiments, chromium is used. The thickness of the features 22 mayrange from 1 nm to 100 nm and may, for example, be about 50 nm. Othermaterials and other thicknesses, however, may be used.

The features 22 may be produced by depositing a layer of suitablematerial that may be etched to produce the pattern 23. In oneembodiment, wafer processing techniques using conventionalphotolithography are used to pattern the features 22. For example,material that is to comprise the pattern 23 is deposited on thesubstrate 18. Photoresist is spun on, and the template 14 is exposedusing a reticle or mask configured to produce the pattern 23 comprisingthe desired features 22. The photoresist is developed and etched to thesubstrate 18, leaving the features 22 adhered to the substrate 18. Thisembodiment may be suitable to print features 22 having pitches and sizeswithin the capabilities of conventional photolithographic systems thatexpose photosensitive material such as photoresist to pattern.

In another embodiment, features 22 that have pitches and sizes beyondthe capability of such conventional photoresist-based photolithographicsystems may be produced, for example, by charged particle beamlithography. For example, some embodiments may utilize focused ion beam(FIB) etching or electron beam lithography (EBL). Charged particle beamlithography advantageously may be used for limited-volume production oftemplates 14, especially those with small-scale features 22. In oneembodiment, FIB lithography may be used to produce a pattern 23comprising an array of 60 nm wide nanowires on a 120 nm pitch. Inanother embodiment, FIB lithography may be used to produce features 22having a width in the range from 10 nm to 80 nm and preferably about 40nm. Features outside these ranges may also be produced.

FIG. 1 depicts one embodiment of the pattern 23. The pattern 23 isintended as a sample and is not intended to limit the scope of thepatterns or features that may be printed by the near-fieldphotolithography system 10. The pattern 23 comprises four substantiallyparallel and substantially rectangular lines 22 a-22 d that areintersected by one substantially rectangular line that ends in aT-shaped region 22 e. The number of lines selected to be illustrated inthe pattern 23 in FIG. 1 is by way of example only and is not intendedto limit the scope of the patterns 23 in the photolithography system 10.The spacing of lines 22 a-22 d is substantially uniform, and the widthof line 22 d is greater than the width of lines 22 a-22 c. The patternillustrated in FIG. 1 also includes four substantially circular dots 22f Patterns 23 with shapes and configurations other than the one depictedin FIG. 1 may be disposed on the substrate 18. Additionally, patterns 23comprising greater or lesser numbers of features 22 and different sizesand spacings than shown in FIG. 1 may be printed.

FIG. 2 is a cross-sectional view along line 2-2 in FIG. 1 that furtherschematically illustrates aspects of the superlens template 14. Light,indicated by arrows 50, is incident on the template 14 and issubstantially transmitted through the substrate 18. The light follows anoptical path defined by the system 10. In the embodiment shown in FIG.2, light is shown as being directed along a straight line that issubstantially perpendicular to the superlens template 14, although thelight may be incident on the superlens template at other angles in otherembodiments. The terms “forward” and “rearward” used herein are madewith respect to this propagation direction of the light along theoptical path through the superlens template 14 to the target (e.g.,photoresist on a semiconductor wafer). For example, a first element maybe said to be forward (rearward) of a second element if the first andsecond elements are disposed such that light propagates through thesecond element before (after) it propagates through the first element.

The superlens template 14 may comprise in addition to the substrate 18and the features 22, a spacer layer 30 and a superlens 34. The superlens34 may be forward of the spacer layer 30, which may be forward of thefeatures 22. FIG. 2 shows sample features 22 a-22 d formed on a surface29 of the substrate 18. The pattern 23 may comprise features 22 having awidth D₁ and a spacing D₂. The length scale D is defined to be D₁+D₂.The length scale D commonly refers to the pitch of the smallest scalefeatures 22 in the pattern 23. In half-pitch imaging, D₁=D₂=D/2.

An advantage of the near-field photolithographic system 10 is theability to reproduce copies of the features 22 that are smaller than thediffraction-limited size W in Eq. (1). For example, the system 10 maysubstantially accurately reproduce images of features 22 having width D₁wherein D₁<W. The width of the features may range from 10 nm to 1000 nm.The spacing between features may also range from 10 nm to 1000 nm. Forexample, in one embodiment of the photolithographic system 10, an arrayof 60-nm wide nanowires may be patterned on a D=120 nm pitch.Accordingly, the spacing between features may be 60 nm.

In some embodiments, the pattern 23 may comprise a periodic array offeatures 22 such as, for example, an array of wires, dots, circles,rings, triangles, or rectangles. The pattern 23 may be in the form of agrating. In other embodiments, the pattern 23 may comprise features 22that are non-periodic or that are a combination of periodic andnon-periodic components. The features 22 may be substantially parallel,as in a periodic array of nanowires, or they may intersect. The pattern23 may comprise features 22 having a symmetry, such as linear,rectangular, or circular. The features 22 may be rounded, circular,triangular, rectangular, rectilinear, or may be irregular in shape. Invarious embodiments, the features 22 may correspond to shapes ofdifferent features formed in layers on a semiconductor wafer. A widerange of other shapes, configurations, arrangements, spacings and sizesare possible.

As shown in FIG. 2, the spacer layer is disposed on the substrate 18 andthe features 22 thereon. After patterning the features 22 on the surface29, for example, the superlens template 14 may be planarized bydepositing the spacer layer 30 on the surface 29 of the substrate 18.The spacer layer 30 comprises a spacer material that is substantiallytransparent to light of the operating wavelength of thephotolithographic system 10. It is advantageous for the spacer materialto have good control of flow characteristics and physical stability. Inaddition, in certain embodiments, the spacer material has an index ofrefraction similar in value to that of the photoresist as discussed morefully below. The spacer material should have the ability to withstandthe formation of the superlens 34, for example the deposition ofmaterial forming the superlens.

Spacer materials may generally comprise nonconductive materials. Theelectromagnetic properties of the spacer material may be characterizedby a permittivity, ∈_(d). In general, the permittivity of a material isa complex number, which has a real part and an imaginary part. Thepermittivity of a medium generally depends on the wavelength of thelight propagating through the medium. For convenience in the descriptionprovided herein, references to a value of a permittivity will be to thereal part unless the imaginary part is explicitly stated for example byreference to the “complex permittivity.” Dielectric materials havepositive permittivities and may be suitable spacer materials. In oneembodiment, the spacer material may be a polymer such as, for example,polymethyl methacrylate (PMMA) or parylene. Other materials such asquartz, glass, or SiO₂ may be used for the spacer layer 30. Forreference, the permittivity of PMMA is approximately 2.4 at a wavelengthof about 365 nm. Other materials and other permittivities may also beused.

In various preferred embodiments, the super lens 34 comprises a layer ofmaterial such as shown in FIG. 2. Moreover, in certain embodiments, thesuperlens 34 comprises a material that may support surface plasmonoscillations at a frequency corresponding to the operating wavelength λof the light used in the photolithographic system 10. Surface plasmonsare charge-density oscillations that propagate along a surface of thesuperlens 34. The amplitude of the oscillation decays exponentially in adirection transverse to the surface of the superlens 34. The interfacebetween the superlens 34 and the spacer layer 30 will be capable ofsupporting surface plasmon oscillations if the permittivity of thespacer material is positive and the permittivity of the superlensmaterial is negative. Accordingly, the superlens 34 comprises materialthat may be characterized by a negative permittivity, ∈_(s). At opticaland ultraviolet frequencies, most metals have negative permittivitiesand may be suitable materials for the superlens 34. In certainembodiments, the imaginary part of the superlens permittivity issufficiently small compared to the absolute value of the real part thepermittivity so that the surface plasmon oscillations do notsubstantially dissipate their energy into heat.

In metals, the permittivity is negative for frequencies smaller than theplasma frequency of electrons in the metal's conduction band.Accordingly, in certain embodiments, the metal used for the superlens 34is such that its plasma frequency exceeds the vacuum frequency of thelight used in the system 10. In some embodiments, noble metals, such as,for example, silver or gold, may be suitable materials for the superlens34, because the collective excitation of conduction electrons enables asurface plasmon oscillation at optical frequencies. In otherembodiments, metals such as, for example, aluminum, copper, chromium, ortantalum, may be used as the superlens 34 material. Other materials mayalso be employed.

The superlens 34 is deposited on the spacer layer 30 by, for example,evaporation, sputtering, chemical deposition or using other techniques.The interface between the surface of the spacer layer 30 and the surfaceof the superlens 34 may be planarized to avoid surface roughness andcorrugations that scatter surface plasmons and distort the imagingcapabilities of the photolithographic system 10. In one embodiment, theroot mean square surface roughness modulation may be below 1 nm for boththe spacer layer 30 and the superlens 34. Smoother or rougher surfaces,however may also be used in different embodiments.

FIG. 3 schematically illustrates further aspects of an embodiment of thephotolithographic system 10. In the embodiment shown, the superlenstemplate 14 is arranged to print images of the features 22 onto aphotosensitive layer 44 disposed on a semiconductor wafer 48.Accordingly, the superlens template 14 is disposed over thesemiconductor wafer 48 and in particular over the photosensitive layer44 on the semiconductor wafer. An intermediary layer 40 is between thesuperlens template 14 and the semiconductor wafer. More particularly,the intermediary layer 40 is disposed between the superlens 34 and thephotosensitive layer 44.

In various embodiments, the intermediary layer 40 comprises a materialsubstantially optically transmissive to the light propagated through thesuperlens template 14. This material may be a fluid or a solid and maybe a dielectric as discussed more fully below.

The photosensitive layer 44 may comprise photoresist that is sensitiveto the operating wavelength propagated through the superlens template14. Conventional semiconductor processing techniques may be used to spinthe photoresist onto the semiconductor wafer 48. In one embodiment, a120 nm thick layer of negative photoresist [NFR105G, Japan SyntheticRubber Microelectronics (JSR Micro)] may be used. Other techniques bothwell known in the art as well as those yet to be discerned may be usedto deposit and/or prepare the photosensitive material.

As shown in FIG. 3, light indicated by arrows 50, is incident on thetemplate 14 and is substantially transmitted through the substrate 18.Light that is incident on one of the features 22 is substantiallyblocked, whereas light that is not incident on a feature 22 propagatesthrough the spacer layer 30, which is substantially transmissive to thelight. The light that propagates beyond the features 22 comprises bothpropagating waves that can reach the far-field and evanescent waves thatare present only in the near-field. Evanescent waves do not diffractlike optical waves in the far-field. Evanescent waves, for example,retain the size and shape of the features 22 more than waves in thefar-field even if the features 22 are on the order of a wavelength orsmaller. Accordingly, in various embodiments, optical energy fromevanescent fields incident on the photosensitive material may have asize and shape similar to that of the features 22. Therefore, evanescentwaves can be said to carry subwavelength information about the features22. The near-field photolithography system 10 uses the evanescent fieldsto expose subwavelength portions of the photosensitive material therebyyielding subwavelength resolution patterning. In this respect, thenear-field photolithography system 10 may capture information present inthe evanescent waves and use this information for high resolutionimaging and patterning.

Since the intensity of evanescent waves decays exponentially withincreasing distance from the features 22, it may be difficult to resolvesubwavelength features 22 if the photosensitive layer 44 is located attoo great a distance from the features 22. For a line array with periodD, the characteristic distance Z over which the intensity of theevanescent waves decays is Z=(D/4π)/√{square root over (1∈_(d)(D²/λ²))},where ∈_(d) is the permittivity of the spacer layer 30. As an example,the decay length in PMMA (Ed 2.4) is 11 nm for 60 nm half-pitch featuresimaged at an operating wavelength of 365 nm. If the photosensitive layer44 is located at too great a distance from the features 22, thesubwavelength information carried by the evanescent waves may be lost,and only features having sizes larger than the diffraction limit in Eq.(1) may be imaged.

Without subscribing to any particular theory, the superlens 34 mayenhance the intensity in the evanescent waves. Optical energy incidenton a rearward surface of the superlens 34 may be coupled into plasmonmodes, which are excited as a result of the incident light. Opticalenergy may be coupled out of a forward surface of the superlens 34 andmay propagate to the photosensitive layer 44. The superlens 34 mayprovide enhanced energy throughput due to resonant excitation of surfaceplasmon oscillations. Resonant plasmon excitation occurs, for example,if the materials comprising the superlens 34 and the spacer layer 30 areselected to have permittivities that are substantially equal and ofopposite sign, that is, ∈_(s)≈−∈_(d). In embodiments of thephotolithographic system 10 in which the spacer layer 30 and thesuperlens 34 comprise materials selected to satisfy this resonantcondition, a broad range of plasmons may be excited by light transmittedby the spacer layer 30 and incident on the superlens 34. This effect isknown as “superlensing.” See, e.g., N. Fang, et al.,“Sub-Diffraction-Limited Optical Imaging with a Silver Superlens,” pp.534-537, Science, Vol. 308, Apr. 22, 2005, which is herein incorporatedby reference in its entirety. Accordingly, the materials comprising thesuperlens 34 and the spacer layer 30 may have permittivities that aresubstantially opposite in sign. These permittivities may also besubstantially equal in magnitude. In these embodiments, subwavelengthinformation carried by the evanescent waves can be used forsub-diffraction-limited imaging. In one embodiment, features 22 withsizes comparable to λ/6 may be resolved.

The thicknesses of the spacer layer 30 and the superlens 34 may beselected so that the template 14 provides superlensing. The thickness ofthe spacer layer 30 may be selected to be in the range 5 nm-200 nm. Ifthe thickness of the spacer layer 30 is many times greater than thedecay length of the evanescent waves, the throughput of thephotolithographic system 10 may be reduced. In one embodiment, thespacer layer 30 may comprise a 40 nm layer of PMMA. Other thicknessesare also possible.

The thickness of the superlens 34 may be selected to be in the range of5 nm-200 nm. Values outside this range may be possible in certainembodiments. The superlens effect, however, can be reduced if thethickness of the superlens 34 is too thick or too thin. Substantialenhancement of evanescent waves may occur if the superlens 34 isselected to have a thickness comparable to the half-pitch size of thefeatures 22 to be imaged or to have a thickness that is a fraction ofthe operating wavelength, for example, λ/10. In one embodiment designedfor imaging an array of 60 nm wires at 120 nm pitch, the superlens 34may be a 35 nm layer of silver. If the superlens 34 is too thick, it mayact as an attenuator of evanescent waves, rather than an amplifier, andthe photolithographic system 10 may be able to resolve only featuresthat are larger than the diffraction-limited size in Eq. (1). Forexample, in an embodiment of the photolithographic system 10 operatingat 365 nm, a superlens 34 that is 120 nm thick may blur features 22smaller than the diffraction limit. See, e.g., S. Durant, et al.,“Comment on ‘Submicron imaging with a planar silver lens’,” p. 4403,Applied Physics Letters, Vol. 84, 2004, which is herein incorporated byreference in its entirety.

Conversely, in various embodiments, the features 22 comprise a materialthat does not possess plasmon resonances near the resonant frequency ofthe spacer layer 30 and superlens 34. Suitable materials for thefeatures 22, for example, may have permittivities that are notsubstantially equal to the permittivity of the superlens material. In anexample embodiment in which the superlens 34 is comprised of silver(∈_(s)≈−2.4 at 365 nm) and the spacer material 30 is comprised of PMMA(∈_(d)≈+2.4), the features 22 advantageously may be comprised ofchromium (∈_(Cr)≈−8.55). The features 22 may comprise other materials aswell.

The surface plasmons excited in the superlens 34 reradiate light thatmay be imaged by the photosensitive layer 44 deposited on thesemiconductor wafer 48. As described above and schematically illustratedin FIG. 3, the intermediary layer 40 is interposed between the superlenstemplate 14 and the photosensitive layer 40. In various embodiments, theintermediary layer 40 is substantially transmissive to light comprisingwavelengths that are transmitted by the superlens 34. The intermediarylayer 40 may also provide a level of protection for the layer ofphotosensitive material 44 from physical contact with the superlenstemplate 14 and may also provide protection to the superlens 34 fromcontact with the photosensitive material 44.

As described above, the intermediary layer 40 may comprise a liquid orsolid layer in certain embodiments. The intermediary layer 40 maycomprise an organic material. Organic materials suitable for use invarious embodiments include, for example, ethyl epoxy propionate (EEP),cyclohexanone, ethyl laurate (EL), propylene glycol monomethyl ether(PGME), or commercially available bottom anti-reflection coatings(BARCs). Additionally, other organic materials may be used. The organicmaterial may comprise a liquid or a solid material in variousembodiments. This organic layer may be chemically compatible with thephotosensitive layer 44 such that the photosensitive material workseffectively. To reduce internal reflections and to improve throughput,the intermediary layer 40 may fill the region between the superlens 34and the photosensitive layer 44 so that there is no air gap betweenthem. The refractive index of the intermediary layer 40 also may besimilar in value to the refractive index of the material comprising thephotosensitive layer 44 to reduce Fresnel reflection. By matching therefractive indices and eliminating air gaps, the intermediary layer 40provides good optical coupling between the superlens 34 and thephotosensitive layer 44 and improves the efficiency of thephotolithographic system 10. Additionally, in certain embodiments, theintermediary layer 40 comprises material that is readily removable fromthe photosensitive layer 44. Organic coatings comprising some or all ofthese desired characteristics are available from many commercialmanufacturers such as, for example, Clariant Corp. (Charlotte, N.C.),Brewer Science, Inc. (Rolla, Mo.), Sigma-Aldrich Co. (St. Louis, Mo.),Shipley Co. (Marlborough, Mass.), or Tokyo Ohka Kogyo Co., Ltd.(Kanagawa, Japan). Other materials may also be used.

Some embodiments of the photolithography system 10 may incorporate asuperlens template 14 that comprises more than one superlens 34. In suchembodiments, the superlenses 34 may be separated from each other byspacer layers 30, which may comprise the same or different dielectricmaterials. In one embodiment, one of the spacer layers 30 comprises again medium for the operating wavelength. The number, thickness, andseparation of the superlenses 34 may be varied so as to reducediffraction and to increase the resolution of the image of the features22. For example, in some embodiments, a single 40 nm superlens may bereplaced by two 20 nm superlenses or four 10 nm superlenses, eachseparated by layers of dielectric material such as PMMA. A variety ofother arrangements and designs are possible.

FIG. 4A schematically illustrates one embodiment of the near-fieldphotolithography system 10. A light source 50 provides light,represented by arrows 54, with operating wavelength λ to form an imageof the features 22 in the photosensitive layer 44 deposited on thesemiconductor wafer 48. The operating wavelength of the light source 50is selected to be suitable for exposing the photosensitive layer 44,which may be comprised of photoresist. The operating wavelength may, ingeneral, range from about 100 nm to about 800 nm, extending from theextreme ultraviolet through the visible part of the electromagneticspectrum. In some embodiments, infrared operating wavelengths may beused.

In certain embodiments, the photolithographic system 10 may userelatively inexpensive high pressure vapor lamps. Various preferredembodiments, for example, may use a high-pressure vapor light source 50that emits light at operating wavelengths of about 365 nm or 580 nm.Alternatively, shorter wavelength light sources, such as, for example,excimer lasers, may be used. However, because the operating wavelengthmay comprise a visible wavelength, light sources that have higherefficiency and lower maintenance costs than excimer laser light sourcesmay be used. Additionally, certain photoresists sensitive to visiblewavelengths such as the mercury I-line are less expensive and morerobust with respect to airborne contaminants than chemically amplifiedresists used in deep ultraviolet photolithography. Other types of lightsources and operating wavelengths may be used as well.

The features 22 may be imaged by exposing the photosensitive layer 44 tolight 54 of the operating wavelength for a predetermined exposure timeand at a predetermined exposure flux. The exposure time and the exposureflux may depend upon the operating wavelength, the photosensitivity ofthe layer 44, the size of the features 22, and the throughput of thesuperlens template 14. The photosensitive layer 44 may be developed andetched using conventional semiconductor wafer processing techniques. Inone embodiment, the template 14 may be flood-exposed to light 54 havinga 365 nm operating wavelength at a flux of 8 mW/cm² for a time of 60seconds. Other operating wavelengths, exposure fluxes, and exposuretimes may be used in other embodiments of the system 10. For example,the exposure time may range from seconds to minutes depending on theexposure flux.

The light 54 is incident on the template 14 so as to producesub-diffraction-limited images as described herein. For example, in oneembodiment, features 22 with widths D₁ as small as λ/6 may be accuratelyreproduced without enlargement or blurring. As shown in the embodimentshown in FIG. 4A, the light source 50 is disposed directly above thesuperlens template 14; however, other configurations may be suitable.For example, the light source 50 may be disposed to one side of thesystem 10 and one or more mirrors, prisms, lenses or other opticalelements may be used to direct the light 54 onto the template 14. Otherconfigurations of the light source 50 are also possible.

The semiconductor wafer 48 is supported by a wafer stage 58, which maybe configured to position the wafer 48. A controller 62 may be used tocontrol the vertical and lateral positioning of the wafer stage 58 withrespect to the superlens template 14. The wafer stage 58 in FIG. 4A maybe configured to translate in both horizontal directions and/or thevertical direction. For example, the wafer stage 58 may be adjusted sothat the photosensitive layer 44 on the semiconductor wafer 48 is in thenear field of the superlens template 14. The controller 62 mayoptionally utilize a feedback system (not shown) to assist inpositioning the wafer stage 58 and in maintaining the photosensitivelayer 44 in the near-field of the superlens 34. The controller 62 maycomprise a computer, computer network, one or more microprocessors, orany electronics or apparatus suitable for controlling the wafer stage58. Servomotors, stepper motors, or piezoelectric-driven devices may beused to move and position the wafer stage 58. In some embodiments, thetemplate 14 may be smaller than the wafer 48, as shown in FIG. 4A, whilein others the template 14 may be of equal or greater size. Accordingly,embodiments of the photolithographic system 10 may be configured forfull-wafer printing, stepping, scanning, or other arrangements.

The superlens template 14 may be supported with respect to thesemiconductor wafer 48 by a support (not shown). The template supportmay be fixed or it may be movable. The controller 62 or a separatetemplate controller (not shown) may be used to control the movement andpositioning of the superlens template 14. In some embodiments, the waferstage 58 may be fixed and the template support may be configured to movethe superlens template 14 with respect to the wafer stage 58. Thetemplate support may for example be adjusted so that the photosensitivelayer 44 on the semiconductor wafer 48 is in the near field of thesuperlens template 14. FIG. 4A shows an embodiment of thephotolithographic system 10 wherein the superlens template 14 ispositioned above the semiconductor wafer 48. In other embodiments, therelative positions of these components may be different. For example, inone embodiment the semiconductor wafer 48 may be secured to the waferstage 58 and disposed above the superlens template 14.

Other configurations are also possible. For example, thephotolithographic system 10 may be configured differently and mayinclude additional components. The order and arrangement of thecomponents may be different and some of the components may be removed.The individual components themselves may be different. For example, awide array of light sources 50, wafer stages 58, and controllers 62 maybe used. The photolithographic system 10 may be configured to use a widevariety of semiconductor wafers 48 and photosensitive layers 44. Thesuperlens template 14 may be arranged and configured differently thanillustrated in FIG. 4A.

In one embodiment illustrated in FIG. 4A, the intermediary layer 40 maycomprise an organic coating deposited on the photosensitive layer 44.The organic coating may comprise a liquid or solid coating in someembodiments. However, the coating need not be organic. The use of acoating, such as an organic coating, as an intermediary layer 40obviates the need for an adhesion-release layer in the photolithographicsystem 10, because the photosensitive layer 44 never makes contact withthe superlens template 14.

In various embodiments, the intermediary layer 40 mediates thepositioning of the photosensitive layer 44 with respect to the superlenstemplate 14. For example, as is well known in the art, current levelingtechnology may be used to provide a substantially constant verticalseparation between the superlens 34 and the photosensitive layer 44. Theintermediary layer 40 may provide a tolerance for placement of thetemplate 14 with respect to the photosensitive material 44 as the waferstage 58 and the template are brought together. The intermediary layer40 may also provide cushioning and may prevent damage as thephotosensitive layer 44 is brought into near-field proximity to thesuperlens 34. As described herein, the intermediary layer 40 may providegood optical coupling between the superlens 34 and the photosensitivelayer 44 so as to increase the efficiency and throughput of thephotolithographic system 10.

The thickness of the intermediary layer 40 should be sufficiently thinso that the photosensitive layer 44 is within the near-field of thesuperlens 34. In some embodiments, the thickness of the intermediarycoating 40 may be in the range of 5 nm-1000 nm. Alternatively, thethickness of the intermediary layer 40 may be selected to be comparablein value to that of the spacer layer 30 or the superlens 34. Forexample, in one embodiment, the intermediary coating 40 may be about 40nm in thickness. In other embodiments, the thickness of the intermediarycoating 40 may be in the range from 5 nm to 200 nm. The coating 40 maybe thicker or thinner in different embodiments. It may be beneficial forthe coating to be easily removable from the photoresist 44. In certainembodiments, for example, the coating may be washed off or may beremoved by stripping with chemical solvents or rinse agents. Othertechniques for removing the coating may also be employed.

In some embodiments, the intermediary coating comprises a plurality oflayers. These layers may have similar properties as described above withrespect to the coating 40 shown in FIG. 4A. For example, the layers mayhave similar refractive indices to reduce reflections. In someembodiments, the different layers may have different properties. Forexample, one of the layers may provide easy release from thephotosensitive material. Other configurations are also possible.

FIG. 4B illustrates an embodiment of the photolithography system 10wherein a liquid flow mechanism may be used to provide the intermediarylayer 40. Many of the components in illustrated in FIG. 4B are generallysimilar in form or function to those illustrated in FIG. 4A and will notbe further discussed except in regard to their differences.

In the embodiment illustrated in FIG. 4B, the intermediary layer 40 isformed by injecting a liquid 80 from an injection nozzle 70 such thatthe liquid 80 flows between the superlens template 14 and thephotoresist-coated semiconductor wafer 48. The liquid 80 is removed byan intake nozzle 74. The intake nozzle 74 may use suction to remove theliquid 80. Although two nozzles 70 and 74 are illustrated in FIG. 4B,other embodiments may use one or more nozzles for both injection andintake. In other embodiments, the intake nozzle 74 may be replaced byone or more drains (not shown) to remove the liquid 80 form the system10. The controller 62 may control the positioning of the wafer stage 58in substantially the same manner as described in FIG. 4A. Optionally,the controller 62 may control the rate at which the liquid 80 isinjected into and removed from the system 10 through the nozzles 70 and74. A separate nozzle controller (not shown) may be used in someembodiments.

The liquid 80 may be stored in a reservoir (not shown) prior toinjection into the system 10 through the injection nozzle 70. In someembodiments, surface tension of the liquid 80 may provide the physicalmechanism through which the liquid spreads in the region between thesuperlens 34 and the photosensitive layer 44. An array of injectionnozzles 70 may also surround the semiconductor wafer 58 to provide aflow of the liquid 80. In some embodiments, the liquid 80 removed by theintake nozzle 74 is recirculated within the system 10, whereas in otherembodiments, the removed liquid 80 is discarded from the system 10. Oneor more pumps (not shown) may be used to inject the liquid 80 into thesystem 10. In some embodiments, the controller 62 may control the pumpsso as to control the rate of flow of the liquid 80 between the superlenstemplate 14 and the semiconductor wafer 48.

The liquid 80 may be used to mediate the transfer of light from thesuperlens 34 to the photosensitive layer 44. Accordingly, the liquid 80may have a refractive index that is similar to the refractive index ofthe material selected for the photosensitive layer 44, e.g., photoresistto reduce reflection. Additionally, the liquid 80 may be chemicallycompatible with the photosensitive layer 44, be of substantially uniformdensity, and be non-contaminating. The liquid 80 may be substantiallytransmissive to light of the wavelengths transmitted by the superlens 34so as to provide sufficient optical throughput of the photolithographicsystem 10. To prevent spurious reflections, the liquid 80 advantageouslymay fill the region between the superlens 34 and the photosensitivelayer 44 so that there are no air gaps.

The liquid 80 may comprise, for example, water, and in particular,purified and de-gassed water. In other embodiments, the liquid 80 maycomprise supercritical carbon dioxide or choline chloride. Other liquidsmay also be used. The liquid 80 may be selected such that itsproperties, such as, for example, viscosity, density, transparency,refractive index, surface tension, thermal conductivity, or thermalcompressibility, are suitable.

A liquid flow system 10 as illustrated in FIG. 4B may be susceptible tothe formation of bubbles in the liquid 80, especially incavitation-prone regions near moving surfaces. Bubbles may result inimage obstructions, aberrations, and anomalies due to light absorption,reflection, or scattering. Accordingly, it is desirable to reduce theincidence of bubble formation in the photolithographic system 10.Optionally, some embodiments may utilize a temperature control system(not shown) and/or a filtration system (not shown) to control theproperties of the liquid 80 so as to improve, e.g., the imagingcapabilities of the photolithographic system 10.

The liquid flow embodiment illustrated in FIG. 4B is commonly known inthe art as a “shower” configuration for immersion photolithographicsystems. Other embodiments of the photolithographic system 10 mayutilize alternative configurations, such as, for example, a “bathtub” or“swimming pool” configuration in which all or some of the system 10 isimmersed or submerged in the liquid 80. In certain embodiments of thephotolithographic system 10, the intermediary layer 40 may comprise acoating on the photosensitive material 44 and a liquid flow mechanism.

The systems and methods described herein advantageously enable thepatterning of semiconductor wafers 48. A photosensitive layer 44comprising photoresist can be exposed, and images of the pattern 23 maybe printed on the semiconductor wafer 48. The photolithographic system10 may be used to pattern metal, semiconductor, and insulating layersand to control doping or alloying of portions of such layers as is wellknown in the art. The systems and methods can be used in a wide range ofother semiconductor device fabrication applications as well. Althoughthe system and methods described herein have been discussed with regardsto photolithographically patterning the semiconductor wafer 48, thesystems and methods may be used in other applications, for example, topattern other types of samples or products. Other types of applicationsare possible, as well.

Advantageously, high-resolution, sub-diffraction-limitedphotolithographic imaging may be provided by the present systems andmethods. However, the systems and methods may be applicable tolow-resolution patterning as well. The photolithographic system 10 mayalso be used with other types of plasmonic lenses. An advantage of thesystems and methods described herein is their simplicity of use in thecommercial fabrication of semiconductor devices.

Various embodiments of the invention have been described above. Althoughthis invention has been described with references to these specificembodiments, the descriptions are intended to be illustrative of theinvention and are not intended to be limiting. Various modifications andapplications may occur to those skilled in the art without departingfrom the true spirit and scope of the invention as defined in theappended claims.

1. A method of fabricating an integrated circuit device on asemiconductor wafer, said method comprising: depositing a material to bepatterned over said semiconductor wafer; depositing a photosensitivelayer on said material to be patterned, said photosensitive layer havinga first index of refraction and responsive to light having a wavelength,λ; disposing a superlens template in an optical path between a lightsource operable to provide said light and said photosensitive layer,said superlens template comprising a plurality of features substantiallyopaque to said light, a dielectric material disposed forward of saidplurality of opaque features, said dielectric material beingsubstantially transmissive to said light, and a superlens disposedforward of said dielectric material; interposing an intermediary layerbetween said superlens and said photosensitive layer to reduce contactof said superlens with said photosensitive layer, the intermediary layercomprising material substantially transmissive to said light, theintermediary layer not permanently in contact with said superlens; anddirecting said light into said superlens template thereby exposingportions of said photosensitive layer to said light.
 2. The method ofclaim 1, said method further comprising: developing said photosensitivelayer; etching said material; and removing said photosensitive layer. 3.The method of claim 1, wherein directing said light into said superlenstemplate thereby exposing portions of said photosensitive layer to saidlight produces a near-field image of said features in or on saidphotosensitive layer.
 4. The method of claim 1, wherein saidphotosensitive layer comprises photoresist.
 5. The method of claim 1,wherein said intermediary layer has a second index of refraction that issubstantially equal to said first index of refraction of saidphotosensitive material.
 6. The method of claim 1, wherein saidsuperlens comprises a layer of metal.
 7. The method of claim 6, whereinsaid metal comprises silver, gold, or titanium.
 8. The method of claim1, wherein said dielectric material comprises polymethyl methacrylate.9. The method of claim 1, wherein said intermediary layer comprises anorganic material.
 10. The method of claim 1, wherein said intermediarylayer comprises a solid.
 11. The method of claim 1, wherein saidintermediary layer comprises a liquid.
 12. The method of claim 11,further comprising providing said liquid from a fluid output port andremoving said liquid from a liquid input port.
 13. The method of claim11, wherein interposing said intermediary layer comprises flowing saidliquid between said superlens and said photosensitive layer.
 14. Themethod of claim 13, further comprising controlling a rate of flow ofsaid liquid between said superlens template and said photosensitivelayer.
 15. The method of claim 11, wherein interposing said intermediarylayer comprises submerging at least a portion of said photosensitivelayer in said liquid.
 16. The method of claim 1, further comprisingproviding said intermediary layer as a coating on said photosensitivelayer.
 17. The method of claim 16, wherein providing said intermediarylayer as a coating comprises depositing said coating on saidphotosensitive layer.
 18. The method of claim 16, wherein said coatingcomprises a plurality of layers.
 19. The method of claim 18, wherein atleast one of said plurality of layers is configured to provide releaseof said coating from said photosensitive layer.
 20. The method of claim16, further comprising removing said coating from said photosensitivelayer after directing said light into said superlens template.
 21. Themethod of claim 20, wherein removing said coating comprises strippingsaid coating with a chemical solvent or a rinse agent.
 22. The method ofclaim 1, wherein interposing said intermediary layer between saidsuperlens and said photosensitive layer further comprises providing asubstantially constant vertical separation between said superlens andsaid photosensitive layer.
 23. The method of claim 1, further comprisingrelatively positioning said photosensitive layer and said superlenstemplate
 24. The method of claim 23, wherein relatively positioning saidphotosensitive layer and said superlens template comprises maintainingsaid photosensitive layer in the near-field of said superlens whiledirecting said light into said superlens template.